Chirping from rear cylinder YICS port

After resuscitating my '82 XZ550, I capped off the YICS ports because I couldn't find a good YICS chamber. I ran it over a year with no problems. I put Stabil in the tank and stored it for two months. Then I checked the air, oil and coolant and took it out for some exercise. It ran fine for 1/2 hour, then developed a chirping or tweeting sound at low rpm's. At first I thought my water pump was at fault so I limped it home. After going over the idling engine with a homemade stethoscope (vacuum tube in my ear) I found the chirping coming from the rear cylinder YICS port. Uncapped, the sound disappeared. Capped, the sound returned. The bike seems a little harder to start and doesn't seem to idle as well as before. Anybody have an idea what is wrong with my motor ?